The present invention relates generally to processing methods for passivating electronic boards, and more particularly, to a method of passivating electronic boards using high density substantially oxygen-free silicon nitride plasmas.
Sealed chip on board (SCOB) processing is a method of coating electronic modules with a thin inorganic passivating layer that provides protection against environmental threats, and which eliminates the need for hermetically packaging the board. The SCOB technology has been developed by the assignee of the present invention, and recent developments have focused on protective coatings of silicon nitride deposited by a low density plasma.
SCOB processed electronic modules require a thin coating of a dielectric material that is deposited uniformly over a relatively large board (typically SAM or SEM(E) size). In order to protect all parts of the circuit, the protective coating must cover portions of bond pads that are hidden by wire bonds. The board is fully populated prior to coating, and the process must not raise the temperature of the board to a temperature that will cause damage. This maximum processing temperature may be as high as 150 degrees Celsius, but a limit of 100 degrees Celsius is normally desired. Also, the processing must not damage semiconductor devices on the board. The protective coating must adhere well to all the surfaces that are to be protected, should be expansion matched so it can pass thermal cycle testing, and should exhibit low stress, preferably compressive.
The general consensus at the assignee of the present invention has been that the requirements for a SCOB passivation coating can only be met by an inorganic material, of which the preferred choice is silicon nitride. Both low frequency, low density plasma chemical vapor deposition processes and high frequency, high density plasma chemical vapor deposition processes allow deposition of insulating films such as silicon nitride or oxide at low temperatures, below 100 degrees Celsius. Ideally, silicon nitride has a formula Si.sub.3 N.sub.4, but coatings usually have an excess of silicon and may contain significant concentrations of oxygen and hydrogen. The effects of Si:N ratio, oxygen and hydrogen concentrations on the effectiveness of the passivation layer are not well characterized.
Large area, low temperature silicon nitride coatings evaluated by the present assignee have been deposited by Ionic Systems, located in Salinas, Calif., using a low density plasma process which is disclosed in U.S. Pat. No. 4,262,631. The oxygen content has been extremely variable, from essentially SiO.sub.2 to an oxygen to silicon ratio of 0.05:3. Silicon oxynitride may be an acceptable passivation coating, but the variation in properties associated with this composition variability is clearly unacceptable in a manufacturing process.
Electron cyclotron resonance (ECR) plasma deposition is generally described in an article by T. Tsuchimoto, J. Vac. Sci. Technol., volume 15, page 1730 (1978). A typical ECR plasma reactor is disclosed in this article. The electron resonance frequency is normally in the microwave range, at about 2.45 GHz, with an 875 Gauss peak magnetic field. Silane is injected into an excited oxygen plasma or into the afterglow thereof. The Tsuchimoto article discusses the deposition of silicon oxide using the ECR plasma reactor.
An article by K. L. Seaward et at. entitled "Role of ions in electron cyclotron resonance plasma-enhanced chemical vapor deposition of silicon dioxide", in J. Vac. Sci. Technol., volume B13, page 118 (1995), compared deposition conditions with film quality and showed that higher quality silicon dioxide films are made at low temperature with high density plasmas. The Seaward et al. study used a reactor from Oxford Plasma Technology, and showed systematic trends in film properties and composition with system variables, including plasma variables. Thus, heretofore, a high density plasma chemical vapor deposition process using an electron cyclotron resonance reactor has been used to deposit silicon dioxide coatings at low temperatures.
An article by Apblett et al. entitled "Silicon nitride growth in a high-density plasma system" in Solid State Technology, November 1995 discusses growth of silicon nitride on surfaces of silicon wafers using a high-density plasma. This article addresses creating surface layers at relatively high processing temperatures compared to the present invention, typically in the range of about 400 degrees Celsius, which is too hot for the application addressed by the present invention. In addition, this article does not address the problem of passivating fully populated electronic boards, which requires relatively low temperatures, typically less than 150 degrees Celsius, and preferably below 100 degrees Celsius.
Accordingly, it is an objective of the present invention to provide for a method of passivating electronic boards using high density substantially oxygen-free silicon nitride plasmas.